1. Field of the Invention
The present invention relates generally to a differential gear permitting a difference in rotational speed between right and left or front and rear driving wheels of a motor vehicle.
2. Description of the Prior Art
Conventional automotive differential gears are commonly of a type in which a pinion gear intervenes between a pair of bevel gears coupled to output shafts and the pinion gear is rotated upon a differential motion which may be caused by an application of external rotational force to the pinion gear shaft, to thereby permit the difference in rotational speed between the output shafts. Another type of differential gears are also known which have a differential motion restricting function for restraining one of the driving wheels from idling at the time of turning or during the traveling on the surface of road having a low frictional coefficient and in which a multiple disk clutch is disposed on the back side of the bevel gear and is pressed by a thrust force of the bevel gear to generate a frictional force, thereby achieving the transmission of a driving force.
For mechanisms such as a multiple disk clutch making use of sliding friction for power transmission, however, it was extremely difficult to limit the frictional force in a half-coupled state to a certain value. At the lower rotational speed, in particular, the differential motion restricting force has disadvantageously suffered from significant instability including an occurrence of so-called stick slip, that is, intermittent generation of static friction and dynamic friction on a clutch-to-clutch basis. Furthermore, an unstable frictional force will result in the generation of noise and vibration, which may adversely affect the running properties.